The present invention relates to pumps usable as implantable ventricular assist devices, to components useful in such pumps, and to methods of using the same.
In certain disease states, the heart lacks sufficient pumping capacity to meet the needs of the body. This inadequacy can be alleviated by providing a mechanical pump referred to as a ventricular assist device to supplement the pumping action of the heart. It would be desirable to provide a ventricular assist device which can be implanted and which can remain in operation for months or years to keep the patient alive while the heart heals, or which can remain in operation permanently during the patient's lifetime if the heart does not heal, or which can keep the patient alive until a suitable donor heart becomes available.
Design of a ventricular assist device presents a daunting engineering challenge. The device must function reliably for the desired period of implantation. Moreover, blood is not a simple fluid, but instead is a complex system containing cells. Severe mechanical action can lead to homolysis, or rupture of the red blood cells, with serious consequences to the patient. Also, blood in contact with an artificial surface, such as the surfaces of a pump, tends to clot. While this tendency can be suppressed to some extent by proper choice of materials, surface finishes and by administration of anticoagulants, it is still important to design the pump so that there are no regions within the device where blood can be trapped or flow is interrupted for relatively prolonged periods. To provide clinically useful assistance to the heart, the device must be capable of delivering a substantial blood flow at a pressure corresponding to normal blood pressure. For example, a ventricular assist device for an adult human patient of normal size should deliver about 1-10 liters per minute of blood at a pressure of about 70-110 mm Hg depending on the needs of the patient.
One type of ventricular assist device or pump uses a balloon. The balloon is placed within the aorta. The balloon is connected to an external pump adapted to repeatedly inflate and deflate the balloon in synchronism with the contractions of the heart muscle to assist the pumping action. Balloon assist devices of this nature have numerous limitations including limited durability and limited capacity.
As described, for example, in U.S. Pat. No. 6,688,861, a miniature electrically-powered rotary pump can be implanted surgically within the patient. Such a pump has a housing with an inlet and an outlet, and a rotor which is suspended within the housing and driven by a rotating magnetic field provided by a stator or winding disposed outside of the housing. During operation, the rotor is suspended within the housing by hydrodynamic and magnetic forces. In such a pump, the rotor may be the only moving part. Because the rotor does not contact the housing during operation, such a pump can operate without wear. Pumps according to the preferred embodiments taught in the '861 patent and related patents have sufficient pumping capacity to provide clinically useful assistance to the heart and can be small enough that they may be implanted within the heart and extend within the patient's thoracic cavity. Pumps of this nature provide numerous advantages including reliability and substantial freedom from hemolysis and thrombogenesis. However, implantation of such a pump involves a majorly invasive surgical procedure.
As described, for example, in Nash, U.S. Pat. No. 4,919,647; Siess, U.S. Pat. No. 7,011,620; and Siess et al., U.S. Pat. No. 7,027,875; as well as in International Patent Publication No. WO 2006/051023, it has been proposed to provide a ventricular assist device in the form of a rotary pump which can be implanted within the vascular system, such as within the aorta during use. Aboul-hosn et al., U.S. Pat. No. 7,022,100, proposes a rotary pump which can be placed within the aorta so that the inlet end of the pump extends through the aortic valve into the left ventricle of the heart.
A ventricular assist device implanted into the vascular system must be extraordinarily compact. For example, each a device typically should have an elongated housing or other element with a diameter or maximum dimension transverse to the direction of elongation less than about 13 mm, and most preferably about 12 mm or less. To meet this constraint, the vascularly-placed ventricular assist devices proposed heretofore resort to mechanically complex arrangements. For example, the device described in U.S. Pat. No. 7,011,620 incorporates an electric motor in an elongated housing. The motor drive shaft extends out of the housing and a seal surrounds the shaft. An impeller is mounted at the distal end of the drive shaft outside of the motor housing and within a separate tubular housing. The pump taught in U.S. Pat. No. 7,022,100 consists of a separate motor using a flexible drive shaft extending through the patient's vascular system to the impeller, with an extraordinarily complex arrangement of seals, bearings, and a circulating pressurized fluid to prevent entry of blood into the flexible shaft. The arrangement taught in WO 2006/051023 and in U.S. Pat. No. 4,919,647 also utilizes flexible shaft drives and external drive motors. These complex systems are susceptible to failure.
Thus, despite very considerable effort devoted in the art heretofore to development of ventricular assist devices, further improvement would be desirable.